Lisette M Bakker, Liesje Mommer, Alexandra J Wright, Jasper van Ruijven
5.3 Material and Methods
5.5.4 Implications and conclusions
Together, our findings suggest that plant diversity did not increase drought resistance. On the one hand, the decreased complementarity effects indicates decreased drought resistance in mixtures, perhaps as a result of decreased soil moisture due to increased transpiration. On the other hand, the mitigation of complementarity effect reductions in high-covered plots, suggest that facilitation via increased cover in mixtures may have increased drought resistance. We did not find an important role of rooting depth in drought resistance, nor did we find evidence for drought-resistance species that increased their performance in mixtures, maintaining community productivity. We suggest that variation in the role of community cover may also explain the differences in the effects of biodiversity on drought resistance between studies (Pfisterer and Schmid, 2002, De Boeck et al., 2008, Isbell et al., 2015, Craven et al., 2016). In studies in which plant diversity increases cover substantially, plant diversity could positively affect drought resistance via microclimate amelioration. In contrast, studies with plant community that have low cover could find decreased drought resistance with species richness. These differential effects of community cover deserve further research.
Differences in the effects of biodiversity on drought resistance among studies may also be due to differences in the extent of the drought conditions. The drought in our study was quite severe: the drought plots did not receive any water for 43 days, a drought that only occurred once in the last 60 years (since daily precipitation measurements were done), while outside conditions were relatively dry and sunny. In contrast, Isbell et al. (2015) used a return time of 10 years in their definition of drought. In addition, soil type may play a role. Our study was done on a sandy soil, which is known to have a low water holding capacity (Rawls et al., 1982, Hudson, 1994) and thus less possibility for facilitation through hydraulic redistribution (Neumann and Cardon, 2012). At medium wetness levels or in soil types with a better holding capacity complementarity effects could be optimized (Neumann and Cardon, 2012, Isbell et al., 2015). Our extreme dry conditions could be the reason that we found no positive effect of biodiversity on resistance, in contrast to the meta-analysis by Isbell et al. (2015). It may also explain why we found a decrease in complementarity effects under drought, while De Boeck et al. (2008), using drought as result of warming (on silt loam), and Craven et al. (2016), including several
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studies with variable results, found no effect. Finally, the extreme conditions in our experiment may also be the reason that the insurance hypothesis did not play a large role in drought resistance in our study. If all species are affected by drought and stop growing, even the drought-resistant species will not be able to compensate for the reduced performance of drought-sensitive species. An important direction for future research is investigating the effect of plant diversity on ecosystem functioning along a drought gradient on different soil types. By manipulating species richness, drought intensity and productivity and cover, we may disentangle the relative contributions of cover, evapotranspiration and species composition along gradients of environmental stress, and further increase our insights into the effect of biodiversity on drought resistance.
5.6 Acknowledgements
The authors would like to greatly thank everyone that helped with the harvest of the experiment, with a special thanks to Natalie Oram and Thomai Kourgiotou. We would like to thank the Laboratory of Geo-information Science and Remote Sensing for borrowing their infra-red camera, and Bert Heusinkveld for a useful discussion and information about drought in the Netherlands. L.M.B. was supported by the Research School for Socio-Economic and Natural Sciences of the Environment (SENSE), the Netherlands. L.M. was supported by a NWO VIDI grant 864.14.006.
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Decreased complementarity effects after an experimental drought
5.7 Supplementary figures
Monocultures Mixtures 0−15 cm 15−30 cm 30−50 cm 50−60 cm 0−15 cm 15−30 cm 30−50 cm 50−60 cm 0 2 4 6 8 Soil layer Moisture (%)Start of the drought
*** *** *** *** *** *** Monocultures Mixtures 0−15 cm 15−30 cm 30−50 cm 50−60 cm 0−15 cm 15−30 cm 30−50 cm 50−60 cm 0 2 4 6 8 Soil layer C D
End of the drought
Figure S5.1 Soil moisture (gravimetric) at the start of the drought (left picture) and at the end of the drought (right picture). At the start of the drought, soil moisture content did not differ between control and drought plots or species richness levels. However, drought strongly reduced soil moisture in all soil layers except for the 50-60 cm layer. Only in control plots, soil moisture was lower in mixtures than in monocultures at the end of the drought (F1, 91 = 6.6, P < 0.05). C = watered control plots (dark grey bars), D = drought plots (light grey bars). Asterisks indicate significance *** = P <0.001 for differences between C and D plots. Bars show means ± standard error.
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Monocultures Mixtures 0 25 50 75 0 25 50 75 30 40 50 Community cover % Av er age comm unity temper ature C D A Monocultures Mixtures 2 4 6 2 4 6 30 40 50Soil moisture (%) at the end of the drought %
Av er age comm unity temper ature C D B Monocultures Mixtures 0 25 50 75 0 25 50 75 5 6 7 8 9 Community cover % Soil moisture(%) at the star t of the drought CD C Monocultures Mixtures 0 25 50 75 0 25 50 75 2 4 6 Community cover % Soil moisture (%)
at the end of the drought
C D D
Figure S5.2 Relationships between community cover, temperature and soil moisture in monocultures and mixtures in drought (D; grey dots) and control (C; black dots) plots.
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Decreased complementarity effects after an experimental drought
Figure S5.3 Species survival in monocultures and 4-species mixtures in drought plots (D; light grey bars) and watered control plots (C; dark grey bars). Nine species, four grasses and five forbs, disappea- red completely from at least one monoculture or mixture. Species disappearance was more common in drought (75% of the disappearances) than in control plots (25%) and in species mixtures (86% of the disappearances) than in monoculture (14%).
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Figure S5.4 Relative complementarity effects (rCE) and deep root fraction community weighted me- ans (DRF CWM) and deep root fraction diversity (DRF FDis) of the plant mixtures were not linked. C = control plots (dark grey dots), D = drought plots (light grey plots), dotted grey line = expected yield based on monoculture yields (rCE=0).